Adaptive lamp monitor with first and second comparison rates

ABSTRACT

A lamp monitor provides a sense resistor for each lamp circuit as a special sense terminal member with a series resistance and sense terminals connected to a circuit board in a lamp monitor module. A multiplexing arrangement allows selection of the circuit to be monitored. The monitor samples the voltage across the series resistance by first connecting a capacitor across the sense terminals to be charged to the voltage and then disconnecting the capacitor from the sense terminals and connecting it across the input of an amplifier. The voltage across the lamps is sensed to provide a ratiometric reference for the digitizing of the amplifier output in an analog to digital converter; and a digital computer corrects the input digital signal with a stored non-linear normalization factor relating current to voltage in the lamps. The normalized value is compared with a predetermined percentage of a learned reference to determine lamp outage; and the learned reference is updated at a slowed rate when no lamp outage is detected.

BACKGROUND OF THE INVENTION

This invention relates to lamp monitor apparatus for determining theoutage of a lamp in a lamp circuit. In particular, this inventionrelates to lamp monitor apparatus suitable for application to a motorvehicle, which typically includes a plurality of lamp circuits havingdifferent numbers and types of lamps, a complicated wiring harness, apossibly noisy electrical power supply and other complicating factors.

One known method of monitoring vehicle lamp circuits takes advantage ofa high degree of symmetry in many of the circuits, which contain lampsin pairs of the same type operated simultaneously for the same purpose.Many vehicle lamps, such as headlamps, taillamps, stoplamps, turn signallamps, etc. are duplicated on two sides of the vehicle. The prior artmethod of lamp monitoring thus provides a series length of wire, knownas "ripcord", of matched impedance in the wiring harness current supplyto each of a pair of lamps and compares the voltage drops across thewires, with a predetermined difference in voltage indicating a lampoutage. This method is adaptive in a sense, in that it uses each lamp asa reference for the other and will thus adapt to changes which affecteach branch of the lamp circuit identically. However, it is somewhatcumbersome, expensive and easily affected by manufacturing variables. Inaddition, it is limited in application, since not all changes affecteach branch of the lamp circuit identically.

SUMMARY OF THE INVENTION

The lamp monitor circuit of this invention is more generally adaptivethan that described above, since it compares each lamp circuit with itsown prior performance to detect comparatively sudden changes as areproduced by a lamp outage but adapts to more gradual changes notassociated with lamp outage. The lamp monitor of this invention alsoprovides accuracy of sense voltage measurement in a vehicle environmentthrough a unique physical structure and measuring circuitry and furtherprovides true compensation for the non-linear lamp voltage/currentrelationship.

A lamp monitor provides a sense resistor for each lamp circuit as aspecial sense terminal member with a series resistance and senseterminals connected to a circuit board in a lamp monitor module. Amultiplexing arrangement allows selection of the circuit to bemonitored. The monitor samples the voltage across the series resistanceby first connecting a capacitor across the sense terminals to be chargedto the voltage and then disconnecting the capacitor from the senseterminals and connecting it across the input of an amplifier. Thevoltage across the lamps is sensed to provide a ratiometric referencefor the digitizing of the amplifier output in an analog to digitalconverter; and a digital computer corrects the input digital signal witha stored non-linear normalization factor relating current to voltage inthe lamps. The normalized value is compared with a predeterminedpercentage of a learned reference to determine lamp outage; and thelearned reference is updated at a slowed rate when no lamp outage isdetected.

Further details and advantages of this invention will be apparent fromthe accompanying drawings and following description of a preferredembodiment.

SUMMARY OF THE DRAWINGS

FIG. 1 shows a lamp system with a lamp monitor according to thisinvention.

FIG. 2 shows the physical structure of a lamp monitor for use in thelamp system of FIG. 1.

FIG. 3 shows a sense resistor/terminal apparatus for use in the lampmonitor of FIG. 2.

FIG. 4 shows a circuit diagram of the lamp monitor of FIG. 2.

FIGS. 5(A), 5(B) and 6 show flow charts describing the operation of thelamp monitor of FIGS. 1-4.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a lamp system with a plurality of lamp circuits, eachconnected to a switched lamp power line through a sense resistor (36,36', 36", 36'"). The lamp system may be a typical motor vehicle lampsystem in which each lamp circuit includes one or more lamps in parallelwith each lamp having one terminal grounded and the other connected backthrough the lamp circuit to the lamp switch and DC electric power. Eachlamp circuit is thus connected through a separate control switch to thecommon electrical power source so that it may be independently activatedand deactivated by its own lamp switch. The system of FIG. 1, however,inserts in each lamp circuit one of the sense resistors 36-36'" betweenthe lamp control switch and the lamps of the circuit in parallel. Theembodiment of FIG. 1 provides four lamp circuits, each with a senseresistor and from one to four lamps; however, a greater or smallernumber of lamp circuits or lamps within each lamp circuit may beprovided within the scope of the invention.

Sense resistors 36-36'"are contained within a lamp monitor module 20which also includes a digital computer 26, a sensing circuit 28, ananalog to digital converter (A/D) 65 and two multiplexers (MUX) 40and 1. MUX 40 has individually selectable inputs each connected to thelamp switch side of one of sense resistors 36-36'"; and MUX 41 hasindividually selectable inputs each connected to the lamp side of senseresistors 36-36'". The outputs of MUX 40 and 41 are connected to sensingcircuit 28, which has an output connected through A/D 65 to computer 26and other control and data lines connected directly to computer 26.Computer 26 has control lines connected to MUX 40 and 41 by whichindividual sense resistors may be selected for connection to sensingcircuit 28.

FIG. 2 shows the physical structure of lamp monitor module 20. A casecomprises a lower member 21 and an upper member 22 with an attached heatsink 24. The case encloses a circuit board 23 having mounted thereon thecomponents of digital computer 26 and sensing circuit 28 and furtherencloses sensing resistors 36-36'" in the form of sense terminal memberssuch as member 15 shown in FIG. 3. Sense terminal member 15 may beformed by simple stamping and bending from sheet stock comprising alloyC71500, made of 70 percent copper and 30 percent nickel with a few traceelements to provide high resistivity and low temperature variation orany similar material. Member 15 comprises series terminals 16 and 17 atthe two ends thereof with a perpendicular sense terminal 18 extendingfrom the base of series terminal 16 and a sense terminal 19 extendingfrom the base of series terminal 17. The bases of series terminals 16and 17, where they join sense terminals 18 and 19, respectively, arejoined by a series resistance element comprising sense resistor 36,which may be straight or provided with curved portions as shown toincrease its effective length as required. Sense resistor 36 isconnected in series with the lamps of a lamp circuit by means of seriesterminals 16 and 18 to provide, by the voltage across sense terminals 18and 19, an indication of the total lamp current flowing to the lamps ofthe lamp circuit. Thus, member 15 is supported in upper case member 22so that sense terminals 18 and 19 are inserted through slots in circuitboard 23 and soldered or otherwise electrically connected to circuittraces thereon connected to MUX 40 and 41. Likewise, series terminals 16and 17 may form part of multiple circuit connectors for the vehiclewiring harness so as to connect the sense resistors into the lampcircuits. The sense resistors 36-36'" are positioned just under heatsink 24 for cooling thereby. The sense resistors for all lamp circuitsmay thus be contained with the sensing circuitry in a single, convenientmodule without the necessity of inserting "ripcords" or addingadditional wires with special connections at remote vehicle locations.

Sense resistor 36 is provided with a resistance which varies with thetype and number of lamps in its lamp circuit to provide, when all lampsare operating normally, a voltage drop of about 100 millivolts. Theactual resistance of sense resistor 36 thus may vary in differentapplications from about 4 milliohms for a high current headlamp circuitto 100 milliohms for a very low current circuit. The sensingresistor/terminal member 15 of FIG. 3 provides a four point voltagemeasurement: that is, the series terminals 16, 17 are outside theresistive region between the sense terminals 18, 19 and thus do notcontribute to the sensed voltage drop. This means that resistancevariations or changes associated with terminals and connectors, such asmisfitting or corroding terminals, are not measured by the senseterminals. This is an important advantage, since the sensed voltage isvery small (100 millivolts) and could be greatly affected by suchchanges.

FIG. 4 shows sensing circuit 28, MUX 40 and 41 and the connections to atypical lamp circuit including sense resistor 36. Lamps 30, 31, 32, 33of a single lamp circuit are connected in parallel to be activated frombattery 35 through sense resistor 36 and a lamp switch 37. Battery 35symbolizes the standard motor vehicle electrical power supply which, inaddition to a battery, also includes an alternator, voltage regulatorand other parts not shown. It provides DC current on demand at an IGNvoltage of 9-16 volts relative to ground. It should be noted that thesensing circuit includes some standard circuit elements, not shown, toprotect the circuit elements from voltage spikes, RFI, etc. Lamp switch37 is any lamp control switch of the vehicle by which a vehicle operatoractivates or deactivates a lamp circuit having one or more lamps. Theembodiment is shown as monitoring a circuit with four parallel lamps;but fewer or more lamps are possible. The upper limit on the number oflamps depends on the tolerance or range of possible lamp currents forlamps on a given circuit. Since a lamp outage is detected by noting thedifference in total lamp current for the circuit with one bulb out, ascompared with the current with all bulbs operating, this difference mustbe greater than the total tolerance of the lamp circuit to beidentifiable as lamp outage. However, the greater the number of parallelconnected lamps in a lamp circuit, the smaller is the difference incurrent when one goes out. Therefore, a smaller total tolerance isrequired to reliably identify a lamp outage in a circuit with a greaternumber of lamps. Four is the greatest number of lamps detectable on acircuit with the standard bulbs used in motor vehicle lamp circuits inthis embodiment. However, tighter bulb tolerances could allow a greaternumber of bulbs in a circuit.

Since it is necessary to monitor each side of sense resistor 36 andthere are a plurality of other sense resistors to be similarlymonitored, the side of sense resistor 36 connected to lamp switch 37 isconnected to one input of a MUX 40; and the side of sense resistor 36connected to lamps 30-33 is also connected to one input of MUX 41. Eachof MUX 40 and 41 have other inputs similarly connected to the othersense resistors and may be operated to connect one of the inputs at atime to an output. The monitor apparatus may thus cycle through the lampcircuits repeatedly with a single sensing circuit. The operation andconnections of such multiplexers are well known and need not be furtherdescribed.

A capacitor 43 is provided for sampling the voltage across a senseresistor by having its leads connected to the outputs of MUX 40 and 41:the upper lead through an analog switch 44 and the lower lead through ananalog switch 45. The analog switches are electronically controlledbetween open and closed states in response to a signal from computer 26.Another analog switch 46 connects the bottom lead of capacitor 43through a resistor 47 (24K) to ground; and an analog switch 48 connectsthe upper lead of capacitor 43 through a resistor 49 (200K) to the inputof an amplifier 50 with a reference resistor 51 (12.1K) to ground and anegative feedback resistor 52 (154K and 3.32K in series) for a totalgain of 14. The output of amplifier 50 is connected through an analogswitch 55 and a unity gain buffer amplifier 60 to the VINT input ofanalog to digital converter (A/D) 65. The input to amplifier 60 isfurnished with a filter comprising a series resistor 56 (200K) and acapacitor 57 (220 pF) to ground. In addition, a resistor 58 (820K) isconnected from the junction of resistor 56 and analog switch 55 toground. The output of amplifier 60 is further connected through aresistor 61 (12K) to IGN: and the VINT input of A/D 65 is provided witha filter comprising a series resistor 62 (4.7K) and a capacitor 63(0.001 uF) to ground.

The output of MUX 41 is further connected through an analog switch 67and a voltage divider comprising resistors 68 (60.4K) and 69 (27.4K) tothe input of a unity gain buffer amplifier 70. The output of amplifier70 is connected through an analog switch 71 and resistor 56 to the inputof amplifier 60 and is further connected through an analog switch 72 tothe input of a buffer amplifier 75 having an output connected to theVREF input of A/D 65. The input of amplifier 75 is provided with afilter comprising a series resistor 73 (24K) and a capacitor 74 (220 pF)to ground. The junction of analog switch 72 and resistor 73 is furtherconnected through a resistor 76 (200K) to a terminal +5V at a regulated5 volts.

A calibration voltage divider comprising a pair of resistors 77 (681Kand 6.34K in series) and 78 (12.1K) connected across input terminal VREFof A/D 65 provides a voltage at the junction of those resistors which isa predetermined percentage of VREF. This junction, filtered with acapacitor 80 (220 pF) to ground, is connected through an analog switch81 to the junction of analog switch 48 and resistor 49. Thus, by closureof analog switches 81 and 55, a calibration voltage is provided to A/D65.

When analog switches 44 and 45 are closed, with all other analogswitches open, to charge capacitor 43 to the voltage across senseresistor 36, the accuracy of the result could be reduced by leakagecurrents through other analog switches which are open but which,nevertheless, are capable of passing small but significant leakagecurrents. These leakage currents could be significant, since capacitor43 only charges to about 100 millivolts but both leads of capacitor 43are at a voltage close to IGN. Such leakage current paths comprise (1)analog switches 48 and 81 and resistor 78 in parallel with capacitor 81to ground, (2) analog switch 46 and resistor 47 to ground, and (3)analog switch 67 and resistors 68 and 69 to ground. The inputs ofamplifiers 50 and 70 provide too high an impedance to allow any leakagecurrents to the remainder of the circuit. To reduce these leakagecurrents to negligible levels, each of the possible leakage currentpaths is provided with a switched connection through a resistor to IGNwhich, when applied to the opposite side of the leaking analog switch,reduces the voltage thereacross and thus the leakage currenttherethrough. For example, analog switch 85 connects the junction ofanalog switch 46 and resistor 47 through a resistor 86 (2.2K) to IGN soas to raise, when closed, the voltage at its end of analog switch 46 toa level near IGN so as to more closely match the expected voltage on thebottom lead of capacitor 43. Likewise, analog switch 88 connects thejunction of analog switch 67 and resistor 68 through a resistor 89(2.2K) to IGN; and analog switches 90 and 91 in series connect thejunction of resistors 77 and 78 through a resistor 92 (2.2K) to IGN.

The operation of the system is seen in the flow charts of FIGS. 5(A),5(B) and 6. FIGS. 5(A) and 5(B) describe the MAIN routine running oncomputer 26, which begins by selecting (100) a desired lamp circuit tomonitor by activating MUX 40 and 41 to connect the sense resistor ofthat lamp circuit to sensing circuit 28. The reduced lamp voltage isthen read six times in step 101. The lamp voltage is defined as thevoltage, referenced to ground, on the lamp side of the sense resistor,as seen by MUX 41; and the reduced lamp voltage is the lamp voltagereduced by a constant factor to fit within a range of five volts. Thus,analog switches 67 and 71 are closed to provide a reduced lamp voltage,comprising the lamp voltage divided by resistors 68 and 69, to the VINTinput of A/D 65, all other analog switches remaining open. Since analogswitch 72 is open, +5 volts is provided to the VREF input of A/D 65 todefine the full scale A/D reference: that is, digital 255 from A/Dcorresponds to 5 volts. The reduction of resistors 68 and 69 serves toreduce the 9-16 volts of IGN applied to the lamps to approximately 2.5to 4.7 volts, so as to be within the full scale range of 5 volts. Thereading is performed 6 times, with the A/D converted, divided lampvoltage read into computer 26 with each reading. Since the A/D referenceis a regulated 5 volts, the divided lamp voltage is an absolutequantity. The computer determines whether the lamp circuit is energizedby determining (102) if all readings are in a voltage range indicatinglamp operation. If any reading is out of range, the lamps are assumed tobe off; and the program returns to step 100 to select a new lamp circuitto monitor. If all six readings are in range, the lamps are assumed tobe turned on (switch 37 closed) and operating in a stable condition.Four more readings are then taken (103) in the same manner and averaged,with the result stored as the divided lamp voltage VDIV. The newreadings are taken in case the lamp voltage was still stabilizing duringthe first of the previous six readings.

The computer next calls (104) subroutine VDIFF four times. Thissubroutine reads the voltage across sense resistor 36, termed thedifferential lamp voltage VDIFF, and corrects the readings for DCoffset. Rather than connect the input of amplifier 50 directly acrosssense resistor 36, capacitor 43 is first charged to the voltage drop ofsense resistor 36; and the capacitor is then connected across the inputof amplifier 50. Use of the two step sample with capacitor 43 eliminatesthe large, variable common mode voltage IGN which would be applied tothe input of amplifier 50 if connected directly to sense resistor 36. Inaddition, capacitor 43 filters the sample as it charges to reduce noisefrom the sample. Subroutine VDIFF, described in greater detail in FIG.6, first connects (110) capacitor 43 across sense resistor 36 so as tobe charged to substantially the same voltage. This is accomplished byclosing analog switches 44 and 45 to connect capacitor 43 acrossresistor 36 and additionally closing analog switches 85, 88, 90 and 91to reduce leakage current through the other analog switches, all ofwhich are open. After analog switches 41, 45, 85, 88, 90 and 91 havebeen left closed for 20 milliseconds (about ten time constants of thecapacitor charging circuit), the analog switches are opened with thecapacitor charged.

The subroutine then reads (111) the capacitor voltage by closing analogswitches 46, 48, 55, 67 and 72 with all others open. The voltage acrosscapacitor 43, amplified by a factor of 14 in amplifier 50, is providedto the VINT input of A/D 65. At the same time, the divided lamp voltageis provided through analog switches 67 and 72 to the VREF input of A/D65 to use as the full scale reference. The use of the divided lampvoltage provides an A/D reference which is ratiometric with the voltageacross the lamps, which may vary with the ignition voltage IGN (exceptfor the non-linear voltage/current relationship in lamps, which will bediscussed below). The lamp voltage is divided to a range of 2.5 to 4.7volts to provide good resolution for the amplified VDIFF at about 1.4volts (14 times 100 millivolts) with a sufficient safety margin toensure that the amplified VDIFF does not exceed full digital scale.

However, there may be a DC offset error associated with amplifier 50.Therefore, a DC offset error correction is provided. As previouslydescribed, VREF is provided across a voltage divider comprisingresistors 77 and 78. A known predetermined percentage of VREF may thusbe provided to A/D 65 through analog switch 81. The subroutine, afterstoring the read value of the voltage across capacitor 43, reads (112) acalibration voltage by opening analog switches 46 and 48 and closinganalog switch 81 (all other analog switches remaining as before) toprovide the known percentage of VREF as the calibration voltage to theVINT input of A/D 65. If the actual output of A/D 65 differs from anexpected value based on the known percentage of VREF, the computercorrects (113) for DC offset by adding this difference to the storedvalue of the capacitor voltage to create the differential lamp voltageVDIFF, which is stored (114).

Returning to step 105 of the MAIN routine after four iterations ofsubroutine VDIFF, an average of the four compensated values of VDIFF isstored; and the lamp voltage is read four more times to ensure that alamp has not been turned off during the running of subroutine VDIFF. Theaverage from step 105 is thus compared (106) with the previous averagefrom step 103. If the averages from these two steps do not agree within5 percent, the program returns to step 100 to select the next lampcircuit. If the averages are within 5 percent, however, the MAIN routinecontinues, as shown in FIG. 5(B).

Next, the stored value of VDIFF is normalized for the actual current inthe lamp circuit. Lamps are supplied with a rated current at a givenrated voltage. However, the relationship between current and voltage isnot linear. A typical relationship may be expressed in the followingequation:

    I.sub.lamp =I.sub.rated (V.sub.app /V.sub.rated).sup.0.55,

wherein the actual lamp current I_(lamp) at an applied lamp voltageV_(app) is related to the rated lamp current and voltage. Therefore,although VDIFF is supposedly ratiometric with the voltage across thelamps, the relationship is not truly ratiometric, due to thisnon-linearity. Normalization compensates for the non-linearcurrent/voltage relationship in the lamps to provide a correction to theratiometric relationship. Thus, computer 26 includes in memory a lookuptable of normalization factors, based on the preceding equation, as afunction of the divided lamp voltage, which corresponds to the appliedvoltage V_(app) in the equation. The program thus normalizes (120) thestored value of VDIFF (which is supposed to vary with lamp current) bymultiplying it by the normalization factor from the lookup tablecorresponding to the stored average divided lamp voltage VDIV.

The normalized value of VDIFF from step 120 is ready for comparison witha reference to determine whether or not a lamp is out in this lampcircuit. However, the reference for each lamp circuit is not fixed butis a learned value allowed to adapt, within certain limits, to actuallamp and circuit conditions. The remainder of the MAIN routine isconcerned with the comparison to determine if a lamp is out and controlof the reference learning process.

After step 120, the routine determines (121) if an OUT flag is alreadyset. An out flag for each lamp circuit comprises a bit in the alterablememory of computer 26 which, when set, indicates a lamp out in the lampcircuit and, when not set, indicates no lamp out in the lamp circuit. Ifthe OUT flag for the lamp circuit is not already set, the routinedetermines (122) if the normalized VDIFF is equal to at least 79 percentof a stored learned reference for the lamp circuit. If not, then VDIFF,which represents the total lamp current of the lamp circuit, is at least21 percent low; and a lamp is probably out. The 79 percent figure isbased on a lamp tolerance of 5 percent. If one lamp of four were to goout, the total current would nominally drop to 75 percent of its formervalue; however, since the lost lamp current might have been smaller thannominal by 5 percent, the test is based on the higher figure of 79percent.

However, additional tests are performed before the lamp outage isaccepted. For example, some vehicles include high resistance side markerlamps which are connected between the ungrounded sides of turn signaland park lamps. In this way the side marker lamps may be energized byeither of the turn signal and park lamp circuits separately by using thelower resistance lamp of the other, unactivated circuit as a groundpath. The other lamp, having a comparatively low resistance, drops acomparatively low voltage and is not lit. It is such a system that, whenboth the park lamp and turn signal circuits are energized, exhibits thephenomenon of side marker lamps which flash in opposite synchronizationwith turn signal lamps. Due to the peculiarities of these side markerlamp connections, the park lamp circuit could be fooled into thinking alamp is out when the side marker lamp is not conducting due to a zerovoltage drop across it when the turn signal is flashing on. Therefore,to prevent such an erroneous conclusion, the routine determines (123) ifthis is a lamp circuit including a side marker lamp and thecorresponding turn signal lamp is on. If so, the routine returns to step100 to select another lamp circuit.

The routine then obtains (124) eight more normalized VDIFF samples andaverages them. This new average is now compared with 79 percent of thelearned reference; and, if it is less, the OUT flag is set (126) and theroutine returned to step 100.

If the OUT flag is determined to be set at step 121, the routine nextdetermines (135) whether the normalized VDIFF is at least equal to 85percent of a constant reference equal to 105 percent of the normalexpected total lamp current for the circuit (five percent higher due tolamp tolerance). This constant is stored as a constant SEED timesanother factor TS which is adjustable to adapt module 20 to differentvehicles and lamp circuits. The module of this embodiment, for example,would have a single constant SEED and a value of TS for each of the fourlamp circuits, which would vary with the number and type of lamps in thecircuits. If the normalized VDIFF is lower, the routine returns to step100. If the normalized VDIFF is equal or higher, eight more normalizedVDIFF values are obtained (136) and averaged and the test is againperformed (137) with the new average and the same result if thenormalized VDIFF is lower than 85 percent of SEED*TS. If the normalizedVDIFF is equal or higher, however, such as after a bad lamp has beenreplaced, the OUT flag is cleared (138) before the routine returns tostep 100.

The learned reference for a lamp circuit is updated only when the lampsare all operating. This is determined at step 122 if the normalizedVDIFF is at least 79 percent of the learned reference. If so, theroutine determines (127) if this is the fourth such determination sincethe last reference update. This slows the reference updating forstability. If this is not the fourth, the routine returns to step 100.If it is, however, the last four normalized VDIFF values are averaged(128); and the average is compared with the learned reference (129). Ifthey are equal, no change will take place; and temporary memory iscleared (132) before the routine returns to step 100. If the new averageis less than the learned reference, the routine ensures that the learnedreference does not fall below a lower limit by determining (130) if thelearned reference is less than a constant comprising 85 percent ofSEED*TS. If it is at this lower limit, then the learned reference is notupdated; and the routine proceeds to step 132. If it is not at the lowerlimit, however, the learned reference is decremented (131) beforeproceeding to step 132. A similar limit of 100 percent SEED*TS exists onthe high side. The routine determines (133) whether the learnedreference is already at the upper limit and increments (134) the learnedreference only if not yet at the upper limit.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. Lamp monitor apparatusfor a vehicle lamp circuit having one or more lamps connected inparallel through a lamp switch to an electric power source, the lampmonitor apparatus comprising, in combination:first means for sensing thevalue of an electrical parameter varying with total lamp current in thelamp circuit when the lamp switch is closed to energize the lamps;second means for repeatedly comparing, at a first rate, the sensed valuewith a first predetermined percentage of a learned reference andsignaling a lamp outage if the sensed value does not exceed the firstpredetermined percentage; and third means for repeatedly comparing, at asecond rate slower than the first rate, the sensed value with thelearned reference and adjusting the learned reference by an incrementtoward the sensed value when no lamp outage is signaled.
 2. The lampmonitor apparatus of claim 1 in which the first predetermined percentageis based substantially on the ratio of total lamp current with one lampnot conducting to the total lamp current with all lamps of the circuitconducting, the ratio increased by a lamp tolerance factor.
 3. The lampmonitor apparatus of claim 1 in which, once a lamp outage is signaled,the second means compares the sensed value with a second predeterminedpercentage of a fixed reference based on the total lamp current with alllamps conducting plus the lamp tolerance factor.
 4. The lamp monitorapparatus of claim 1 in which the first means senses the electricalparameter a plurality of times and the sensed value is an averagethereof.
 5. The lamp monitor apparatus of claim 1 in which the firstmeans comprises an electrical resistor connected in series with thelamps so as to carry the total lamp current.
 6. The lamp monitorapparatus of claim 5 in which the electrical parameter is derived fromthe voltage drop across the electrical resistor.
 7. The lamp monitorapparatus of claim 1 in which the vehicle lamp circuit is associated ina vehicle with one or more additional vehicle lamp circuits, the firstmeans comprises separate elements associated with the lamp circuit andeach of the additional lamp circuits, and the second and third meanscomprise a single apparatus selectively connected by multiplex apparatusto each of the separate elements of the first means.
 8. The lamp monitorapparatus of claim 1 in which the third means limits adjustment of thelearned reference between low and high limits.
 9. The lamp monitorapparatus of claim 1 further comprising:fourth means for sensing thevoltage across the one or more lamps in parallel; and fifth means fordetermining whether the lamp switch is closed to energize the lamps bycomparing the sensed voltage of the one or more lamps in parallel with alamp voltage reference indicative of an operating voltage across thelamps.
 10. The lamp monitor apparatus of claim 9 in which:the fifthmeans compares the sensed voltage across the one or more lamps inparallel with the lamp voltage reference a plurality of times; and apredetermined number of the plurality of comparisons indicative of anoperating voltage across the lamps is required to indicate that the lampswitch is closed.
 11. The lamp monitor apparatus of claim 10 in whichthe predetermined number of the plurality of comparisons is the entireplurality of comparisons.
 12. The lamp monitor apparatus of claim 10 inwhich the fifth means performs a first number of the plurality ofcomparisons before the second means compares the sensed value of theelectrical parameter with the first predetermined percentage of thefirst reference and a second number of the plurality of comparisonsafter the second means compares the sensed value of the electricalparameter with the first predetermined percentage of the first referencebut before the second means signals a lamp outage.
 13. The lamp monitorapparatus of claim 1 in which the second means and third means are partsof a single digital system in which the second means performs itscomparisons at regular first predetermined times and the third meansperforms its comparisons at second predetermined times associated withnon-consecutive occurrences of the first predetermined times.
 14. Thelamp monitor apparatus of claim 13 in which:the single digital system isa digital computer, the second means and third means comprise elementsthereof controlled by first and second portions, respectively, of acontrol program, and the second portion of the control program isexecuted no more often than once every N executions of the first portionof the control program, where N is an integer.
 15. The lamp monitorapparatus of claim 14 in which N equals 4.